In recent years, a point-to-multipoint access optical communication system referred to as a passive optical network (PON) system constructed with a public network that employs an optical fiber has been widely used in access networks for providing multimedia services to individual households.
The PON system is made up of one station-side optical line terminal (OLT) and multiple subscriber-side optical network units (ONUs) that are connected to the OLT via an optical star coupler. The PON system provides several advantages in that: a reduction in the operational costs can be expected since multiple ONUs can share the OLT as well as the majority of the optical fiber which serves as transmission line; outdoor installation is easy since the optical star coupler, which serves as a passive component, does not require a supply of power; and the system is highly reliable. Accordingly, the PON system recently has been actively introduced as an optical communication system for realizing broadband networks.
For example, in a G-PON system having a transmission speed of 2.5 Gbit/s downstream and 1.25 Gbit/s upstream which is standardized compliant with the international standard G.984 series of the ITU-T, a downstream from the OLT to the ONUs employs a broadcast system using an optical wavelength band of 1480 to 1500 nm. Each of the ONUs retrieves only the data of the allotted time slot from the optical signal sent by the OLT. On the other hand, an upstream from each of the ONUs to the OLT uses an optical wavelength band of 1290 to 1330 nm and employs a time-division multiplex communication system for controlling transmission timing such that data sent from the ONUs does not collide with each other. Since the timing of the transmission is not constant and since there are quiescent periods between data sent by each of the ONUs, the signal received by the OLT is a burst optical signal.
Also, in an XG-PON system having a transmission speed of 10 Gbit/s downstream and 2.5 Gbit/s upstream, which is standardized compliant with the international standard G.987 series of the ITU-T, a downstream from the OLT to the ONUs employs a broadcast system using an optical wavelength band of 1575 to 1580 nm. Each of the ONUs retrieves only the data of the allotted time slot from the optical signal sent by the OLT. On the other hand, an upstream from each of the ONUs to the OLT uses an optical wavelength band of 1260 to 1280 nm and employs a time-division multiplex communication system for controlling transmission timing such that data from the ONUs do not collide with each other.
In such PON systems, since each of the ONUs are positioned at different distances from the OLT, the light-receiving level in the OLT of the optical signal transmitted by each of the ONUs differs per receiving packet received by the OLT from each of the ONUs. Accordingly, there is a demand for an optical receiver at OLT having a wide dynamic range that provides stable and high-speed regeneration of packets with different light-receiving levels. Therefore, optical receivers for OLT are provided with an automatic gain control (AGC) circuit that causes the conversion gain of a trans-impedance amplifier, which converts photoelectric current into a voltage signal, to quickly change to an appropriate gain in accordance with the light-receiving level.
Since the AGC circuit has a time constant for the conversion gain to converge after start of packet signal receiving, a predetermined time is necessary for the OLT optical receiver to regenerate data stably after start of packet signal receiving. Here, the time required for the conversion gain to converge is limited by the transmission speed of the system. In the case of a G-PON system or an XG-PON system, it is necessary to cause the conversion gain to converge within several tens of ns, and there is a demand for a high-speed AGC function.
Here, each packet signal contains an overhead area and a data area, and the overhead area is a fixed digit string of alternating “zeros” and “ones” whereas the data area is a string of random digits. The AGC function of an OLT optical receiver ideally operates to perform convergence in the overhead area at high speed and maintain a constant gain in the data area.
Various systems have been proposed for AGC circuits that have a high-speed response characteristic, and that stabilize at the appropriate gain in a data area (Patent Literature 1, for example). The AGC circuit described in Patent Literature 1 functions to control the conversion min based on the search results of a peak level detection circuit, and functions to make the response time of the AGC circuit high speed only for the vicinity of the head of a received packet signal.